Fluid transfer devices are known that operate in a first direction as a pump and in a second direction as a motor. These devices may comprise a housing within which a rotor rotates with respect to a port plate and a cam plate angled with respect to the rotor's axis of rotation. The rotor includes one or more bores (generally an odd number) each for receiving a piston. One end of each piston held in contact with the cam plate. As the rotor rotates with respect to the housing, each piston moves axially with respect to the rotor and the port plate.
The port plate includes a fluid inlet through which a fluid enters the housing when a piston aligned with the fluid inlet moves away from the port plate and a fluid outlet through which fluid exits the housing when a piston aligned with the fluid outlet moves toward the port plate. When the rotor is connected to a source of motive power, the rotation of the rotor causes the pistons to draw fluid from the inlet and expel fluid through the outlet; when operated in this manner, the fluid transfer device is referred to as an axial piston pump. When fluid is applied under pressure to the fluid inlet and drawn from the fluid outlet at a lower pressure, the rotor is caused to turn by the pressure difference; when operated in this manner, the fluid transfer device is referred to as a hydraulic motor. Thus “axial piston pump” and “hydraulic motor” may refer to the same fluid transfer device, depending on the what is making the rotor turn.
It is known to provide rotor bores with a ceramic or metal liner to improve wear resistance and achieve satisfactory tribological performance. These liners may be thin-walled, right circularly cylindrical tubes that are shrink fitted into typically metallic rotors. A shrink fit, press fit, braze or similar type of connection is desirable to retain the liner in the bore under typical operation. Unfortunately, non-symmetrical stresses to the liner can be imparted during installation (or occur during use) and these stresses can crack the brittle ceramic bore liners.